The present invention relates to an optical isolator which is improved so as to suppress generation of noises.
There has been used, in the recent high speed large capacity communication system, an optical communication system and a semiconductor laser as a light source. In the optical communication system, a light beam emitted from such a semiconductor laser as a light beam to be transmitted for communication is projected on the end face of a fiber through a relay lens and propagates in the fiber. In this case, a part of the transmission light is sometimes reflected by end faces of the fiber and lenses and often returned back to the light source as a returned light beam. The returned light beam generates noises since the oscillation of the semiconductor laser is disturbed when the direction of polarization of the returned light beam is identical to that of the transmission light.
In order to inhibit the generation of any noise, the machinery and tools for optical communication each, in general, comprises an optical isolator directly connected to a light source. The conventional optical isolator generally comprises a hollow cylindrical magnet whose ends are both opened and in which a Faraday's rotator is disposed; a polarizer and an analyzer arranged on the sides of the magnet. Anti-reflection films are formed on these Faraday's rotator, polarizer and analyzer. The Faraday's rotator can turns the polarization direction of a light beam to be transmitted for 45.degree. due to the action of the magnetic field established by the magnet. Moreover, the polarization directions of the polarizer and the analyzer are also turned an angle of 45.degree. relative to one another so as to be in agreement with the rotational angle. A transmission light beam emitted from the light source is first polarized by the polarizer, then the polarization direction thereof is turned 45.degree. by the action of the Faraday's rotator and the light beam passes through the analyzer at an angle of 45.degree. with respect to the polarization direction of the polarizer. In this case, some returned light beams can pass through the analyzer, but the polarization direction thereof is again turned 45.degree. by the Faraday's rotator. For this reason, the overall polarized angle of the returned light is equal to 90.degree., thus the polarization direction of the returned light is perpendicular to that of the transmission light beam and accordingly, any propagation of the returned light beam is inhibited by the polarizer.
Japanese Utility Model Application Publication No. 1-43873 discloses a semiconductor laser device which comprises a semiconductor laser which emits a linearly polarized light beam and to which garnet is directly fitted as a Faraday's rotator and a collimater lens or condenser lens arranged ahead of the laser tube. Moreover, the technique proposed in Japanese Patent Provisional Publication No. 63-84185 makes use of a Faraday's rotator as a material for the window of a semiconductor laser and is designed in such a manner that a light beam passes through an analyzer after the passage through a lens. In the technique disclosed in Japanese Patent Provisional Publication No. 3-174789, a semiconductor laser is equipped with a cap which comprises a Faraday's rotator and an analyzer. However, this structure suffers from various problems such that the angle adjustment is required even after the semiconductor laser and the cap are integrated and that the cap does not show an ability of completely shielding any reflected light depending on the methods for the connection thereof to the laser.
There has been strongly desired for the miniaturization of optical isolators and the reduction of the production cost thereof. For this reason, the sizes of polarizers and analyzers are also substantially reduced. The same is true for the collimater lens used in the foregoing semiconductor laser device. In case of the conventional optical isolator, the polarization direction of a light beam transmitting through a polarizer should be correctly adjusted with respect to the polarization direction of the light beam transmitting through an analyzer. In this respect, it is very difficult to directly adjusting the position of a quite fine part. Moreover, the polarizer is fine in size, but is not cheap at all and such a built-in polarizer substantially increases the size of the optical isolator.
Japanese Utility Model Application Publication No. 1-43873 discloses a semiconductor laser device which comprises a semiconductor laser having a linear polarization plane and an optical isolator which are continuously formed without any clear distinction. However, it is very difficult to directly adjusting the angle of the collimater lens with respect to the polarization direction of the transmission light beam. This makes the mass-production thereof in an industrial scale difficult.
In the technique disclosed in Japanese Patent Provisional Publication No. 63-84185, a diffusing light beam is incident upon a magneto-optical element. Therefore, the magneto-optical element must have a large diameter. This leads to an increase in the cost for the materials required and correspondingly, makes the resulting isolator unit expensive. Besides, the structure or shape as shown in the attached figure of this article makes the angle-control of the analyzer difficult. In general, the angle of a polarization-optical system is adjusted while making the reflected light incident upon the optical system in order to achieve desired functions of the optical isolator, but the adjustment becomes impossible when the semiconductor laser is equipped with the Faraday's rotator. Further, the technique disclosed in Japanese Patent Provisional Publication No. 3-174789 requires the use of a magneto-optical element having a large diameter, but this leads to an increase in the cost for the materials required and correspondingly, makes the resulting isolator unit expensive. In addition, it is difficult to fit the isolator unit to the semiconductor device and to perform the alignment thereof.